Small horses liked it hot

Artist's reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left)
that stands about 5 feet high at the shoulders and weighs approximately 1000 lbs. Sifrhippus was the size of a small house cat (about 8 lbs.) at the beginning of the Eocene (~55.8 million years ago) and is the earliest known horse.
[Illustration by Danielle Byerley, Florida Museum of Natural History]

The earliest horses were closer in size to a housecat than to the modern-day animals we're familiar with. Even at this small scale, the body size of these ancient horses evolved over time. New research shows that environmental temperatures drove these changes.

It's well-known that animals tend to be smaller in hot climates and larger in cold ones. This pattern is known as Bergmann's rule and is found in nearly 2/3 of all mammals. But, is this trend related to temperature, because large animals conserve more body heat than small ones? Or, is it caused by some other ecological factor such as food availability? Scientists haven't been sure.

To learn more about the evolution of body size, Ross Secord of the University of Nebraska, Lincoln and the University of Florida and colleagues studied a collection of fossil horse teeth from Wyoming. The ages of these teeth spanned a period of major climate fluctuation, including the Paleocene-Eocene Thermal Maximum, or PETM, which occurred about 55 million years ago.

The researchers analyzed the size of the fossil teeth and also the chemistry, since oxygen and carbon isotopes can be clues to what the environment was like when the teeth were growing. The results showed that the early horse Sifrhippus sandrae initially had a body size of about 5.6 kilograms but then shrank by approximately 30 percent at the start of the PETM, as climate warmed. Then, it grew abruptly at the end of the PETM, when climate cooled, reaching about 7 kilograms. (By comparison, modern-day horses weigh hundreds of kilograms.)

The fossils' chemistry also suggested that the climate was becoming wetter while Sifrhiuppus' body size was decreasing, which likely meant there were more plants to eat. The researchers therefore concluded that temperature, and not productivity or food availability, was the main driver behind the evolution of body size.